Indium gallium nitride-based ohmic contact layers for gallium nitride-based devices

ABSTRACT

Light emitting devices include a gallium nitride-based epitaxial structure that includes an active light emitting region and a gallium nitride-based outer layer, for example gallium nitride. A indium nitride-based layer, such as indium gallium nitride, is provided directly on the outer layer. A reflective metal layer or a transparent conductive oxide layer is provided directly on the indium gallium nitride layer opposite the outer layer. The indium gallium nitride layer forms a direct ohmic contact with the outer layer. An ohmic metal layer need not be used. Related fabrication methods are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application Ser. No.61/040,400, filed Mar. 28, 2008, entitled Indium Nitride-Based OhmicContact Layers for Gallium Nitride-Based Devices, assigned to theassignee of the present application, the disclosure of which is herebyincorporated herein by reference in its entirety as if set forth fullyherein.

FIELD OF THE INVENTION

This invention relates to semiconductor devices, such as light emittingdevices, and methods of fabricating same, and more particularly to ohmiccontacts for gallium nitride-based devices and methods of fabricatingsame.

BACKGROUND OF THE INVENTION

Light emitting diodes and laser diodes are well known solid stateelectronic devices capable of generating light upon application of asufficient voltage. Light emitting diodes and laser diodes may begenerally referred to as light emitting devices (“LEDs”). Light emittingdevices generally include a p-n junction formed in an epitaxial layergrown on a substrate such as sapphire, silicon, silicon carbide, galliumarsenide and the like. The wavelength distribution of the lightgenerated by the LED generally depends on the material from which thep-n junction is fabricated and the structure of the thin epitaxiallayers that make up the active light emitting region of the device.

Typically, an LED chip includes a substrate, an N-type epitaxial regionformed on the substrate and a P-type epitaxial region formed on theN-type epitaxial region (or vice-versa). In order to facilitate theapplication of a voltage to the device, an anode ohmic contact is formedon a P-type region of the device (typically, an exposed P-type epitaxiallayer) and a cathode ohmic contact is formed on an N-type region of thedevice (such as the substrate or an exposed N-type epitaxial layer). Theohmic contact may typically include an ohmic metal layer, such asplatinum, palladium, nickel, titanium, gold, tin or combinationsthereof. The ohmic metal layer is generally provided to reduce theforward or turn-on voltage of the device.

SUMMARY OF THE INVENTION

Light emitting devices according to some embodiments of the presentinvention include a gallium nitride-based epitaxial structure thatincludes an active light emitting region and a gallium nitride-basedouter layer, for example binary gallium nitride. The active lightemitting region may include multiple quantum well structures with indiumgallium nitride-based wells. An indium gallium nitride-based layer isprovided directly on the gallium nitride-based outer layer. In someembodiments, the indium gallium nitride-based layer has a higherpercentage indium than the wells. A reflective metal layer is provideddirectly on the indium gallium nitride-based layer opposite the galliumnitride-based outer layer. However, in other embodiments, an ohmic metallayer is also provided between the indium gallium nitride-based layerand the reflective metal layer.

In some embodiments, the indium gallium nitride-based layer is undoped(i.e., not intentionally doped) and the gallium nitride-based outerlayer is doped P-type. In other embodiments, the indium galliumnitride-based layer may be between about 5 Å and about 100 Å thick. Instill other embodiments, the gallium nitride-based outer layer mayinclude a nonplanar outer surface, and the indium gallium nitride-basedlayer is also nonplanar. In some embodiments, the reflective metal layercomprises silver, nickel-silver alloy and/or aluminum. Other embodimentsalso include a barrier layer on the reflective metal layer opposite theindium gallium nitride-based layer, and a bonding layer on the barrierlayer opposite the reflective metal layer. Yet other embodiments mayinclude a silicon carbide or other substrate on the galliumnitride-based epitaxial structure opposite the outer layer.

Light emitting devices according to other embodiments of the inventioninclude a gallium nitride-based epitaxial structure that includes anactive light emitting region and a gallium nitride-based outer layer,for example binary gallium nitride. The active light emitting region mayinclude multiple quantum well structures with indium galliumnitride-based wells. An indium gallium nitride-based layer is provideddirectly on the gallium nitride-based outer layer. In some embodiments,the indium gallium nitride-based layer has a higher percentage indiumthan the wells. A transparent conductive spacer layer is provideddirectly on the indium gallium nitride-based layer opposite the galliumnitride-based outer layer. A reflective metal layer is provided directlyon the transparent conductive spacer layer opposite the indium galliumnitride-based layer. Dopings, thicknesses, barrier layers, bondinglayers, nonplanar layers and/or substrates may be provided as wasdescribed above. In some embodiments, the transparent conductive spacerlayer may be sufficiently thick to space the active light emittingregion apart from the reflective metal layer, so as to increase, and insome embodiments maximize, reflection of light from the reflectivelayer.

Light emitting devices according to yet other embodiments of the presentinvention include a gallium nitride-based epitaxial structure thatincludes an active light emitting region and a gallium nitride-basedouter layer, for example binary gallium nitride. A binary indium nitridelayer is provided directly on the gallium nitride-based outer layer. Areflective metal layer or a transparent conductive oxide layer isprovided directly on the binary indium nitride layer opposite thegallium nitride-based outer layer. Accordingly, a binary indium nitridelayer forms a direct ohmic contact with a gallium nitride-based outerlayer. An ohmic metal layer need not be used, although it may beprovided between the binary indium nitride layer and the galliumnitride-based outer layer in other embodiments.

In still other embodiments of the present invention, the reflectivemetal layer may comprise silver, nickel and/or aluminum, and thetransparent conductive oxide layer may comprise indium tin oxide.Moreover, some embodiments may provide a barrier layer on the reflectivemetal layer opposite the binary indium nitride layer and a bonding layeron the barrier layer opposite the reflective metal layer. Otherembodiments may provide a bond pad on the transparent conductive oxidelayer opposite the binary indium nitride layer. Moreover, a siliconcarbide or other substrate may be provided on the gallium nitride-basedepitaxial structure opposite the outer layer.

Light emitting devices according to still other embodiments of theinvention include a gallium nitride-based epitaxial structure thatincludes an active light emitting region. An indium galliumnitride-based layer including therein clusters of elemental indiumand/or binary indium nitride is provided on the gallium nitride-basedepitaxial structure. A reflective metal layer or a transparentconductive oxide layer is provided on the indium gallium nitride-basedlayer opposite the gallium nitride-based epitaxial structure. In someembodiments, the indium gallium nitride-based layer including thereinclusters of elemental indium and/or binary indium nitride is an indiumgallium nitride layer including therein clusters of elemental indiumand/or binary indium nitride. Remaining layers in the device includingthe reflective metal layer, the transparent conductive oxide layer, thebarrier layer, the bond pad and/or the substrate may be provided as wasdescribed above.

Embodiments of the present invention have been described above inconnection with light emitting devices. However, analogous methods offorming light emitting devices may also be provided according to otherembodiments of the present invention. In some embodiments, a galliumnitride-based structure that includes an active light emitting regionand a gallium nitride-based outer layer, for example binary galliumnitride, may be epitaxially formed. An indium gallium nitride layer maybe epitaxially formed directly on the outer layer. A reflective metallayer or a transparent conductive layer may be formed directly on theindium gallium nitride layer. The reflective layer may be formed byelectron beam and/or sputter deposition without annealing, or asubsequent anneal may take place. The gallium nitride-based outer layermay be fabricated by exposing the gallium nitride-based structure tosources of gallium, nitrogen and a P-type dopant, and the indium galliumnitride layer may be formed by further exposing the galliumnitride-based structure to the sources of gallium and nitrogen, and to asource of indium, while terminating exposure to the P-type dopant. Otherstructures may be formed as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a conventional flip-chip lightemitting device mounted on a carrier such as a submount.

FIG. 2 is a cross-sectional view of a conventional non-flip-chip lightemitting device.

FIGS. 3 and 5 are cross-sectional views of flip-chip light emittingdevices according to various embodiments of the present inventionmounted on carriers such as submounts.

FIGS. 4, 6 and 7 are cross-sectional views of non-flip-chip lightemitting devices according to various embodiments of the presentinvention.

FIGS. 8, 9, 10, 11 and 12 are cross-sectional views of flip-chip lightemitting devices according to various other embodiments of the presentinvention mounted on carriers, such as submounts.

DETAILED DESCRIPTION

The present invention now will be described more fully with reference tothe accompanying drawings, in which embodiments of the invention areshown. This invention may, however, be embodied in many different formsand should not be construed as limited to the embodiments set forthherein. Rather, these embodiments are provided so that this disclosurewill be thorough and complete, and will fully convey the scope of theinvention to those skilled in the art. In the drawings, the size andrelative sizes of layers and regions may be exaggerated for clarity.Like numbers refer to like elements throughout.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” another element, it can bedirectly on the other element or intervening elements may also bepresent. It will be understood that if part of an element is referred toas “outer,” it is closer to the outside of the device than other partsof the element. Furthermore, relative terms such as “beneath” or“overlies” may be used herein to describe a relationship of one layer orregion to another layer or region relative to a substrate or base layeras illustrated in the figures. It will be understood that these termsare intended to encompass different orientations of the device inaddition to the orientation depicted in the figures. Finally, the term“directly” means that there are no intervening elements. As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items and may be abbreviated as “/”.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, components, regions, layersand/or sections, these elements, components, regions, layers and/orsections should not be limited by these terms. These terms are only usedto distinguish one element, component, region, layer or section fromanother region, layer or section. Thus, a first element, component,region, layer or section discussed below could be termed a secondelement, component, region, layer or section without departing from theteachings of the present invention.

Embodiments of the invention are described herein with reference tocross-sectional illustrations that are schematic illustrations ofidealized embodiments of the invention. As such, variations from theshapes of the illustrations as a result, for example, of manufacturingtechniques and/or tolerances, are to be expected. Thus, embodiments ofthe invention should not be construed as limited to the particularshapes of regions illustrated herein but are to include deviations inshapes that result, for example, from manufacturing. For example, aregion illustrated or described as a rectangle will, typically, haverounded or curved features due to normal manufacturing tolerances. Thus,the regions illustrated in the figures are schematic in nature and theirshapes are not intended to illustrate the precise shape of a region of adevice and are not intended to limit the scope of the invention.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andthis specification and will not be interpreted in an idealized or overlyformal sense unless expressly so defined herein.

Various embodiments of semiconductor light emitting devices will bedescribed herein. As used herein, the term “semiconductor light emittingdevice” may include a light emitting diode, laser diode and/or othersemiconductor device which includes one or more semiconductor lightemitting layers, which may include silicon, silicon carbide, galliumnitride and/or other semiconductor materials. A light emitting devicemay or may not include a substrate such as a sapphire, silicon, siliconcarbide, aluminum nitride and/or other microelectronic substrate(s). Alight emitting device may include one or more contact layers which mayinclude metal and/or other conductive layers. In some embodiments,ultraviolet, blue and/or green light emitting diodes may be provided.Red and/or amber LEDs may also be provided. The design and fabricationof semiconductor light emitting devices are understood by those havingskill in the art and need not be described in detail herein. Forexample, the semiconductor light emitting devices may be galliumnitride-based light emitting diodes or lasers fabricated on a siliconcarbide substrate such as those devices manufactured and sold by Cree,Inc. of Durham, N.C.

Finally, when referring to a Group III nitride compound, the word“-based” means that additional Group III elements may be included in thecompound. Thus, for example, a gallium nitride-based layer includesbinary, ternary, quaternary, etc. compounds that include gallium nitride(GaN), and an indium gallium nitride-based layer includes quaternary andhigher compounds that include indium gallium nitride (InGaN). Incontrast, the terms “elemental”, “binary”, “ternary”, “quaternary”, etc.mean that additional Group III materials are not included in thecompound. Thus, for example, binary gallium nitride excludes ternary,quaternary, etc. compounds, and ternary indium gallium nitride excludesquaternary, etc. compounds. However, all of the above defined terms donot preclude the Group III nitride compound from being doped P-typeand/or N-type using, for example, P-type dopants such as magnesiumand/or N-type dopants such as silicon.

Conventional LEDs based on Group III nitrides typically use an ohmicmetal layer between the device layer and (1) a reflective metal layer inthe case of flip-chip LEDs, or (2) a metal bond pad in the case ofconventional (i.e., non-flip-chip) LEDs. The ohmic metal layer mayinclude platinum, palladium, titanium, gold, tin or combinationsthereof. Without the ohmic metal layer, the device may suffer from highforward or turn-on voltage (Vf).

Some embodiments of the present invention may arise from a recognitionthat, even though ohmic metal layers are generally used in lightemitting devices, the ohmic metal layer still may produce an undesirablyhigh forward voltage. Moreover, an ohmic metal layer may undesirablyabsorb at least some emission from the active layer, even if the ohmicmetal layer is made very thin. Some embodiments of the present inventioncan eliminate the need for a separate ohmic metal layer, or can expandthe range of metals that can form ohmic contacts to nitride-baseddevices. Moreover, embodiments of the invention may also be used with anohmic metal layer to further reduce the forward voltage.

Some embodiments of the present invention can provide an indiumnitride-based layer, such as an indium gallium nitride-based layer,directly on a gallium nitride-based outer layer, for example binarygallium nitride, and a reflective metal layer or a transparentconductive oxide layer directly on the indium nitride-based layer. Theindium nitride-based layer, such as an indium gallium nitride-basedlayer, can provide a low bandgap contact layer that is undoped (i.e.,not intentionally doped) or is intentionally doped N-type or P-type. Forflip-chip mounting, a reflective metal layer may be provided directly onthe indium nitride-based ohmic contact layer. For standard devices(i.e., non-flip-chip), there is no need to cover the surface of theindium nitride-based contact layer with a metal ohmic contact. In eithercase, forward voltage of the resulting device may be reduced, and lightemission may be increased.

Stated differently, a reflective metal layer, such as silver, does notprovide a good ohmic contact to conventional gallium nitride-based LEDs.However, some embodiments of the present invention provide anintermediate layer of indium nitride-based material, such as indiumgallium nitride-based material, that can allow a good ohmic contact tobe formed, so that the need for a separate ohmic metal layer may beeliminated. Moreover, when the indium nitride-based layer, such as theindium gallium nitride-based layer, is directly on a galliumnitride-based outer layer, for example binary gallium nitride, an evenbetter ohmic contact may be provided.

FIG. 1 is a cross-sectional view of a conventional flip-chip LED. Asshown in FIG. 1, a conventional flip-chip LED 10 includes a galliumnitride-based epitaxial structure, also referred to as “LED epi” 28 thatincludes an active light emitting region 11, a gallium nitride-basedouter layer 12 of, for example, binary gallium nitride that may be dopedP-type, also referred to as a P—GaN-based layer and a galliumnitride-based layer 18 of, for example, binary gallium nitride that maybe doped N-type, also referred to as an N—GaN-based layer. An ohmicmetal layer 13 is provided on the P—GaN-based layer 12 and an ohmicmetal layer 23 is provided on the N—GaN-based layer 18. The ohmic metallayers 13/23 may include platinum, palladium, nickel, titanium, gold,tin and/or combinations thereof. A reflective metal layer 14, forexample comprising silver, nickel silver alloy and/or aluminum isprovided on the ohmic metal layer 13. A barrier layer 15, comprising,for example, platinum, is provided on the reflective metal layer 14. Abonding layer 16, for example comprising gold, is provided on thebarrier layer 15 and is used to bond the device to a carrier 17, such asa submount. A substrate may be provided on the active region 11 in someembodiments.

FIG. 2 is a cross-sectional view of a conventional non-flip-chip LED 20that includes a substrate 21, an LED epi region 28, including an activeregion 11, a P—GaN-based outer layer 12 and an N—GaN-based layer 18, aswas described above, and an ohmic metal layer 13 as was described above.The ohmic layer 13 provides current spreading over the entire topsurface of the device. A bond pad 22, such as a nickel/gold bond pad, isprovided on the ohmic metal layer 13.

FIG. 3 is a cross-sectional view of a flip-chip light emitting deviceaccording to various embodiments of the present invention. As shown inFIG. 3, these devices 30 may include an active region 11, a P—GaN-basedlayer 12, a reflective metal layer 14, a barrier layer 15, a bondinglayer 16 and a carrier 17, as was described above in connection withFIG. 1. The active region 11 also may generally include a multi-quantumwell (MQW) structure that includes indium gallium nitride-based wellsand gallium nitride-based barriers. A binary indium nitride layer 33 isprovided directly on the gallium nitride-based outer layer 12. Moreover,the reflective metal layer 14 is provided directly on the binary indiumnitride layer 33 opposite the gallium nitride-based outer layer 12. Aseparate ohmic metal layer 13 is not provided. However, in otherembodiments, an ohmic metal layer may be provided to further decreasethe forward voltage. The active region 11, the outer layer 12 and thebinary indium nitride layer 33 may be fabricated as an LED epitaxialstructure 38.

Still referring to FIG. 3, an N-type gallium nitride-based layer 18 maybe provided on the active region 11 opposite the P—GaN-based layer 12,and may also be included as part of the LED epi structure 38. TheN—GaN-based layer 18 may be provided on a substrate 19, which maycomprise silicon carbide, sapphire and/or other transparent substrates.The substrate 19 is shown in dashed line because it can be removed afterforming the LED epi structure 38 on the substrate 19, so that it neednot be present in the final structure.

Moreover, when the N—GaN-based layer 18 and/or the substrate 19 areincluded, one or more surfaces thereof may be roughened to allowimproved light extraction. Thus, the interface between the N—GaN-basedlayer 18 and the substrate 19 may be nonplanar (e.g., textured and/orroughened) and/or the outer surface of the substrate 19 remote from theN—GaN-based layer 18 may be nonplanar. Nonplanar surfaces may beprovided by adjusting growth parameters and/or by texturing/roughening aplanar surface after it is grown.

FIG. 4 illustrates a non-flip-chip LED according to various embodimentsof the present invention. As shown in FIG. 4, these non-flip-chip LEDs40 may include a substrate 21, an active region 11, an N—GaN-based layer18, an outer layer 12 and a bond pad 22, as was described above inconnection with FIG. 2. However, a binary indium nitride layer 43 isprovided directly on the P—GaN-based outer layer 12. Moreover, atransparent conductive oxide layer 44, such as indium tin oxide (ITO),is provided directly on the binary indium nitride layer 43 opposite theouter layer 12, to provide the requisite current spreading. A separateohmic metal layer 13 is not provided. However, it may be provided inother embodiments. The active region 11, the GaN-based layers 12 and 13,and the binary indium nitride layer 43 may be fabricated as an LEDepitaxial structure 48.

The binary indium nitride layer 33 and/or 43 of FIGS. 3 and/or 4,respectively, can provide a lower bandgap than the outer P—GaN-basedlayer 12 of these figures, and can also provide relatively lowresistivity, to thereby provide a good ohmic contact.

Additional discussion of the binary indium nitride layers 33, 43 willnow be provided. In some embodiments, these layers are undoped, i.e.,not intentionally doped. These layers may have a thickness of betweenabout 5 Å and about 100 Å, and, in some embodiments, about 20 Å. Inother embodiments, the binary indium nitride layer 33, 43 may beintentionally doped N-type or P-type. These layers may typically beN-type as grown. However, N-type dopants, such as silicon, may also beused. For P-type layers, magnesium (Mg) may be used as a dopant.However, magnesium doped layers may also be N-type if the amount ofmagnesium is not sufficient for the layer to be P-type. The binaryindium nitride layer 33, 43 may be grown at the same conditions as wellsin the active region 11. The binary indium nitride-based layer may bepit-free or may manifest pits as measured by an atomic force microscopeor scanning electron microscope.

Moreover, in FIG. 3, the reflective metal layer 14 may be deposited sothat it is ohmic in its as-deposited state. For example, electron beamdeposition and/or sputter deposition may be used. Alternatively, thereflective metal 14 may be non-ohmic as deposited and then may beannealed, for example between about 200° C. and about 700° and, in someembodiments, at about 300° C., for between about 1 minute and 30minutes, and, in some embodiments, for about 15 minutes, to make thereflective metal 14 ohmically contact the binary indium nitride layer33.

FIGS. 5 and 6 illustrate other flip-chip and non-flip-chip embodimentsof the present invention, respectively. In FIGS. 5 and 6, these LEDs 50and 60 include an indium gallium nitride-based layer 53 that includestherein clusters 54 of elemental indium and/or binary indium nitride.Growth conditions may be controlled, for example by decreasing thegrowth temperature to less than about 750° C., to provide an indiumgallium nitride-based layer 53 with clusters 54 of elemental indiumand/or binary indium nitride therein. These clusters 54 may be ofaverage size of less than about 100 Å, and, in some embodiments, lessthan about 50 Å in diameter. These clusters 54 may have even lowerbandgaps, such as a bandgap of less than about 1 eV, and in someembodiments, of about 0.7 eV, and may provide conductive paths that canfurther reduce overall forward voltage. These layers may also have anindium composition of between about 5 atomic percent and about 30 atomicpercent. Other parameters of the indium gallium nitride-based layer 53may be as was described above for layers 33 and/or 43.

It will also be understood that other embodiments of FIGS. 5 and 6 neednot provide the indium gallium nitride-based layer 53 including thereinclusters 54 of elemental indium and/or binary indium nitride directly ona binary gallium nitride layer 12 or directly on an active region 11.Moreover, still other embodiments of FIGS. 5 and 6 need not provide thereflective metal layer 14 or the ITO layer 44 directly on the indiumgallium nitride-based layer 53 including therein clusters 54 ofelemental indium and/or binary indium nitride.

FIG. 7 is a cross-sectional view of non-flip-chip LEDs according toother embodiments of the present invention. These non-flip-chip LEDs 70include a silicon carbide substrate 21 that may be 4 H or 6 H N-typesilicon carbide. In other embodiments, the substrate 21 can includesapphire, bulk gallium nitride, bulk aluminum nitride or other suitablesubstrates. In some embodiments, the substrate 21 can be a growthsubstrate on which the epitaxial layers 78 forming the LED structure 70are formed. In other embodiments, the substrate 21 can be a carriersubstrate to which the epitaxial layers 78 are transferred. In otherembodiments, the substrate 21 can be removed altogether, as isunderstood in the art.

Still referring to FIG. 7, the LED epi structure 78 includes a silicondoped binary GaN layer 75 on the substrate 21. One or more buffer layers76, such as a silicon doped ternary aluminum gallium nitride bufferlayer, may be provided between the substrate 21 and the binary GaN layer75. An N-type superlattice (SLS) structure 74 can be formed on thebinary GaN layer 75. For example, 25 repetitions of ternary indiumgallium nitride/binary gallium nitride may be provided. A multi-quantumwell (MQW) 73 may be formed on the superlattice 74. In some embodiments,the MQW may comprise 6 to 8 repetitions of ternary indium galliumnitride/binary gallium nitride. An undoped quaternary aluminum indiumgallium nitride layer 72 may be formed on the MQW 73, and a ternaryaluminum gallium nitride layer 71 doped with a P-type dopant, such asmagnesium, may be formed on the quaternary aluminum indium galliumnitride layer 72. A gallium nitride-based layer, such as a binarygallium nitride layer 12 also doped with a P-type dopant, such asmagnesium, may be formed on the ternary aluminum gallium nitride layer71. The active region of the device 70 may include the GaN:Si layer 75,the superlattice 74 and the multi-quantum well 73. An InN-based layer33, 43, 53 is provided on the GaN-based layer 12 as was described above.An ITO layer 44 and a bond pad 22 are provided as was described above.

FIG. 8 is a cross-sectional view of a flip-chip light emitting deviceaccording to other embodiments of the present invention. As shown inFIG. 8, these devices 80 may include an active region 11, a P—GaN-basedouter layer 12, a reflective metal layer 14, a barrier layer 15, abonding layer 16 and a carrier 17, as was described above, for example,in connection with FIG. 3. An N—GaN-based layer 18 and/or a substrate 19also may be provided. An undoped indium gallium nitride-based layer 83is provided directly on the gallium nitride-based outer layer 12.Moreover, the reflective metal layer 14 is provided directly on theundoped indium gallium nitride-based layer 83. A separate ohmic metallayer 13 is not provided, although it may be provided in otherembodiments. The active region 11, the outer layer 12, and the undopedindium gallium nitride-based layer 83 may be fabricated as an LEDepitaxial structure 88.

More specifically, the indium gallium nitride-based layer 83 may have athickness of between about 5 Å and about 100 Å in some embodiments, and,in some embodiments, may be about 20 Å thick. An indium composition ofabout 5 atomic percent to about 30 atomic percent may be provided, and,in some embodiments, about 10 atomic percent indium may be provided. Inother embodiments, the indium composition in the indium galliumnitride-based layer 83 may be equal to or greater than the indiumcomposition of the wells of the active region 11. Moreover, the indiumgallium nitride-based layer 83 may be undoped, i.e., not intentionallydoped. More specifically, N- and P-doping sources may not be used duringthe epitaxial growth of the indium gallium nitride-based layer 83. Itwill be understood that some dopants may still be found in the indiumgallium nitride-based layer 83 due to, for example, residual dopants inthe epitaxial system, so that an actual doping of magnesium from about1×10¹⁵ cm⁻³ to about 1×10²² cm⁻³ may be present in some embodiments,and, in some embodiments, a magnesium doping level of about 1×10¹⁷ cm⁻³may be found. In other embodiments, the layer 83 may be intentionallydoped N- or P-type.

The undoped indium gallium nitride-based layer 83 may be fabricateddirectly on the P—GaN-based outer layer 12 by continuing to expose theLED epi structure 88 to a source of gallium and a source of nitrogenwhile turning on the source of indium and also turning off the source ofmagnesium or other P-type dopants.

The indium gallium nitride-based layer 83 may further act to decreasethe bandgap, without being unduly absorptive. Accordingly, the thicknessand/or indium content of the indium gallium nitride-based layer 83 maybe selected to provide acceptable electrical conduction withoutexcessively absorbing emitted radiation from the active region 11.Accordingly, in some embodiments, an indium gallium nitride layer thatis about 20 Å in thickness, having about 10 atomic percent weight indiumand not being intentionally doped may satisfy these criteria. In otherembodiments, the indium gallium nitride-based layer 83 may be made eventhinner, so as to not be unduly absorptive, even though higher indiumcontent than the indium gallium nitride quantum wells of the activeregion is provided. Thus, if the indium gallium nitride wells of theactive region 11 have an indium content of between about 20% and 40%, aneven higher indium content may be provided in the undoped indium galliumnitride-based layer 83, provided that this layer is sufficiently thin soas to not excessively absorb emitted radiation from the active region.

FIG. 9 is a cross-sectional view of a flip-chip light emitting deviceaccording to still other embodiments of the present invention. Thesedevices 90 may be similar to devices 80 of FIG. 8, except that the LEDepi structure 98 includes a P—GaN-based layer 92 that has a nonplanar(e.g., roughened or textured) surface, illustrated by a wavy line inFIG. 9, and the undoped indium gallium nitride-based layer 93 is alsononplanar. Thus, a conformal nonplanar layer of undoped indium galliumnitride-based material 93 may be provided on the nonplanar surface ofthe P—GaN-based layer 92. It will be understood that in FIG. 9, thethicknesses of the layers are not drawn to scale, and that layer 93 istypically much thinner than layer 92, so that layer 93 may be regardedas a thin conformal coating on P—GaN-based layer 92. The nonplanarsurface of P—GaN-based layer 92 may enhance extraction of light, and theprovision of the nonplanar undoped InGaN layer 93 may compensate for thevoltage drop that may otherwise be presented by the P—GaN-based outerlayer 92. Accordingly, improved light extraction may be obtained withoutthe need to pay a voltage penalty. It will be understood that thenonplanar layers 12 and/or 93 may be grown in a nonplanar form byadjusting the growth parameters and/or planar layers may benonplanarized or further nonplanarized after growth.

FIG. 10 is a cross-sectional view of a flip-chip light emitting deviceaccording to still other embodiments of the present invention. As shownin FIG. 10, these devices 100 may include an LED epi region 88/98, aswas described above in connection with FIGS. 8 and/or 9. However, atransparent conductive spacer 104 is added directly on the undopedInGaN-based layer 83/93, and the reflective metal layer 14 is provideddirectly on the transparent conductive spacer 104. The transparentconductive spacer 104, which may comprise a transparent conductive oxidelayer, such as indium tin oxide (ITO), zinc oxide (ZnO), or nickel oxide(NiO), having a thickness of between about 5 Å and about 5,000 Å may beprovided.

The transparent conductive spacer layer 104 may perform a uniquefunction in embodiments of FIG. 10. Typically, transparent conductiveoxides are used to provide current spreading, while allowing light topass therethrough. However, in embodiments of FIG. 10, current spreadingmay not be needed, because the undoped InGaN-based layer 83/93 providesa good ohmic contact. Nonetheless, by providing a transparent conductivelayer 104, the efficiency of the reflective metal layer 14 may beincreased and, in some embodiments, optimized, by providing a desired,and in some embodiments optimum, spacing between the active region 11and the reflective metal layer 14.

Stated differently, by moving the reflective metal 14 further away fromthe active region 11, an increased, and in some embodiments maximized,percentage of reflection may be provided. The thickness of thetransparent conductive layer may selected based on the distance betweenthe active region 11 and the reflective metal layer 14, the compositionof the reflective metal layer 14, the frequency of light that is beingemitted by the active region 11 and/or other parameters, to increase ormaximize the reflection from the reflective metal layer 14. In someembodiments, the spacing may be selected as a function of the wavelengthof the light, to reduce destructive interference between the incomingand reflected light, and thereby provide enhanced or maximum reflectedlight. In some embodiments, a spacing of one quarter the wavelength oflight, or multiples of one quarter the wavelength of the emitted light,may be used. Thus, a transparent conductive oxide, such as ITO, may beused as a transparent conductive spacer in a flip-chip LED, to enhancethe efficiency of the reflective metal layer 14. It will be understoodthat embodiments of FIG. 10 may be used in combination with thenonplanar P—GaN layer 92 and nonplanar undoped InGaN layer 93 of FIG. 9.

FIGS. 11 and 12 are cross-sectional views of flip-chip light emittingdevices according to yet other embodiments of the invention. The devices110 of FIG. 11 may be similar to devices 80 of FIG. 8, and the devices120 of FIG. 12 may be similar to the devices 90 of FIG. 9, except thatan ohmic metal layer 113, 123, respectively, is added between theundoped InGaN layer 83/93, and the reflective metal layer 14. The ohmicmetal layer 113/123 may comprise nickel. As shown in embodiments ofFIGS. 11 and 12, the addition of an ohmic metal layer may furtherdecrease the forward voltage of the light emitting device when used incombination with the undoped InGaN layer 83/93.

Embodiments of the invention have been described above in connectionwith light emitting devices. However, ohmic contact structures asdescribed herein may also be used for gallium nitride-based devices thatare not light emitting. For example, contact structures as describedherein may be used in connection with gallium nitride-based HighElectron Mobility Transistor (HEMT) devices. In HEMT devices, layers 12and 33 of FIG. 3 may be used on an active HEMT region and layers 12 and53 of FIG. 5 may be used on an active HEMT region. Accordingly, galliumnitride-based devices that are not light emitting may also be providedaccording to other embodiments of the present invention.

In the drawings and specification, there have been disclosed embodimentsof the invention and, although specific terms are employed, they areused in a generic and descriptive sense only and not for purposes oflimitation, the scope of the invention being set forth in the followingclaims.

1. A light emitting device comprising: a gallium nitride-based epitaxial structure that includes an active light emitting region and a gallium nitride-based outer layer; an undoped indium gallium nitride-based layer directly on the gallium nitride-based outer layer; and a reflective metal layer directly on the undoped indium gallium nitride-based layer opposite the gallium nitride-based outer layer.
 2. A light emitting device according to claim 1 wherein the gallium nitride-based outer layer is doped P-type.
 3. A light emitting device according to claim 1 wherein the reflective metal layer comprises silver, nickel-silver alloy and/or aluminum.
 4. A light emitting device according to claim 1 wherein the undoped indium gallium nitride-based layer is between about 5 Å and about 100 Å thick.
 5. A light emitting device according to claim 1 wherein the gallium nitride-based outer layer includes a nonplanar outer surface and wherein the undoped indium gallium nitride-based layer is also nonplanar.
 6. A light emitting device according to claim 1 further comprising a barrier layer on the reflective metal layer opposite the undoped indium gallium nitride-based layer and a bonding layer on the barrier layer opposite the reflective metal layer.
 7. A light emitting device according to claim 1 further comprising a substrate on the gallium nitride-based epitaxial structure opposite the outer layer.
 8. A light emitting device comprising: a gallium nitride-based epitaxial structure that includes an active light emitting region and a gallium nitride-based outer layer; an undoped indium gallium nitride-based layer directly on the gallium nitride-based outer layer; a transparent conductive spacer layer directly on the undoped indium gallium nitride-based layer opposite the gallium nitride-based outer layer; and a reflective metal layer directly on the transparent conductive spacer layer opposite the undoped indium gallium nitride-based layer.
 9. A light emitting device according to claim 8 wherein the gallium nitride-based layer is doped P-type.
 10. A light emitting device according to claim 8 wherein the reflective metal layer comprises silver, nickel-silver alloy and/or aluminum.
 11. A light emitting device according to claim 8 wherein the transparent conductive spacer layer comprises a transparent conductive oxide layer.
 12. A light emitting device according to claim 8 wherein the undoped indium gallium nitride-based layer is between about 5 Å and about 100 Å thick.
 13. A light emitting device according to claim 8 wherein the gallium nitride-based outer layer includes a nonplanar outer surface and wherein the undoped indium gallium nitride-based layer is also nonplanar.
 14. A light emitting device according to claim 8 further comprising a barrier layer on the reflective metal layer opposite the undoped indium gallium nitride-based layer and a bonding layer on the barrier layer opposite the reflective metal layer.
 15. A light emitting device of claim 8 wherein the transparent conductive spacer layer is sufficiently thick to space the active light emitting region away from the reflective metal layer so as to increase reflection of light from the reflective metal layer.
 16. A light emitting device according to claim 8 further comprising a substrate on the gallium nitride-based epitaxial structure opposite the outer layer.
 17. A light emitting device comprising: a gallium nitride-based epitaxial structure that includes an active light emitting region and a gallium nitride-based outer layer; a binary indium nitride layer directly on the gallium nitride-based outer layer; and a reflective metal layer or a transparent conductive oxide layer directly on the binary indium nitride layer opposite the gallium nitride-based outer layer.
 18. A light emitting device according to claim 17 wherein the reflective metal layer comprises silver, nickel-silver alloy and/or aluminum.
 19. A light emitting device according to claim 17 wherein the transparent conductive oxide layer comprises indium tin oxide.
 20. A light emitting device according to claim 17 further comprising a barrier layer on the reflective metal layer opposite the binary indium nitride layer and a bonding layer on the barrier layer opposite the reflective metal layer.
 21. A light emitting device according to claim 17 further comprising a bond pad on the transparent conductive oxide layer opposite the binary indium nitride layer.
 22. A light emitting device according to claim 17 further comprising a substrate on the gallium nitride-based epitaxial structure opposite the outer layer.
 23. A light emitting device comprising: a gallium nitride-based epitaxial structure that includes an active light emitting region; an indium gallium nitride-based layer including therein clusters of elemental indium and/or binary indium nitride, on the gallium nitride-based epitaxial structure; and a reflective metal layer or a transparent conductive oxide layer on the indium gallium nitride-based layer opposite the gallium nitride-based epitaxial structure.
 24. A light emitting device according to claim 23 wherein the indium gallium nitride-based layer including therein clusters of elemental indium and/or binary indium nitride is directly on the gallium nitride-based epitaxial structure and wherein the reflective metal layer or the transparent conductive oxide layer is directly on the indium gallium nitride-based layer opposite the gallium nitride-based epitaxial structure.
 25. A light emitting device according to claim 23 wherein indium gallium nitride-based layer including therein clusters of elemental indium and/or binary indium nitride is a ternary indium gallium nitride layer including therein clusters of elemental indium and/or binary indium nitride.
 26. A light emitting device according to claim 23 wherein the reflective metal layer comprises silver, nickel-silver alloy and/or aluminum.
 27. A light emitting device according to claim 23 wherein the transparent conductive oxide layer comprises indium tin oxide.
 28. A light emitting device according to claim 23 further comprising a barrier layer on the reflective metal layer opposite the indium nitride-based layer and a bonding layer on the barrier layer opposite the reflective metal layer.
 29. A light emitting device according to claim 23 further comprising a bond pad on the transparent conductive oxide layer opposite the indium gallium nitride-based layer.
 30. A light emitting device according to claim 23 further comprising a substrate on the gallium nitride-based epitaxial structure opposite the indium gallium nitride-based layer.
 31. A method of fabricating a light emitting device comprising: epitaxially forming a gallium nitride-based structure that includes an active light emitting region and a gallium nitride-based outer layer; epitaxially forming an undoped indium gallium nitride layer directly on the gallium nitride-based outer layer; and forming a reflective metal layer or a transparent conductive layer directly on the undoped indium gallium nitride layer.
 32. A method according to claim 31 wherein epitaxially forming a gallium nitride-based outer layer comprises exposing the gallium nitride-based structure to sources of gallium, nitrogen and a P-type dopant and wherein epitaxially forming an undoped indium gallium nitride layer comprises further exposing the gallium nitride-based structure to the sources of gallium and nitrogen, and to a source of indium, while terminating exposure to the P-type dopant. 